Stereoscopic 3-D imaging requires the presentation of two slightly different sets of images to a viewer; one set corresponds to a left eye viewpoint and the other corresponds to a right eye viewpoint. When the sets of images are presented so that only the left eye of a viewer can see the left eye set of images and the right eye can only see the right eye set of images, the viewer will be able to perceive a 3-D image.
Several different methods of separating left and right eye images are known. In the anaglyph method, different colour filters are used. Typically, the left eye and right eye images are projected simultaneously but in different colours, say red and blue respectively, and the viewer wears a pair of glasses fitted with red and blue filters arranged to appropriately separate the images. A major disadvantage of this method is that the resulting 3-D images are deficient in colour information.
Another method of image separation involves the use of mutually extinguishing polarizing filters. The filters are placed in front of left and right eye projectors with their polarizing axes at 90 degrees to each other. Viewers wear eyeglasses with polarizing filters arranged in the same orientation as the filters on the projectors. The left and right eye images appear on the screen at the same time, but only the left eye polarized light is transmitted through the left eye lens of the eyeglasses and only the right eye polarized light is transmitted through the right eye lens. This method is inexpensive and allows full colour 3-D images. However, it has limitations in that a substantial amount of unwanted transmission can occur and can result in the formation of objectionable ghost images. For instance, the polarization characteristics of the light can be significantly altered by reflection from a screen, though metallic screen coatings will mitigate this effect. If linear polarizers (which are most effective) are used, ghost images will also increase as the viewer tilts his or her head to the left or right.
A third known method involves time multiplexing of left and right eye images. Left and right eye images are presented alternately so that there is only one eye image on the screen at any one moment in time. Viewers wear glasses which alternately block the view of one eye so that only the correct image will be seen by each eye. In other words when a left eye image is projected onto a screen the left eye lens of the glasses will be transparent and the right eye lens will be opaque. When the image on the screen changes to a right eye image, the left lens of the glasses becomes opaque and the right eye lens becomes transparent. The glasses typically have electro-optic liquid crystal shutters and are powered by batteries. This method largely overcomes the problems of unwanted transmission due to head tilt and does not require a special screen to maintain polarization; however it is substantially more expensive than each of the other methods of image separation described previously. An additional disadvantage is that electronic alternate eye glasses weigh considerably more than passive glasses. This is a significant problem when the glasses are designed with large oversized lenses for use in a wide field of view motion picture theatre such as those marketed by Imax Corporation.
The liquid crystal shutters that are used in time-multiplexing stereoscopic imaging are usually extinguishing shutters made of at least two linear polarizers on either side of a liquid crystal cell which contains a thin layer liquid crystal material between two sheets of glass. The two polarizers are oriented with their axes generally orthogonal and the liquid crystal material acts as a variable polarizer influenced by an electric field. Such shutters block a significant proportion of the light when in an opaque state but they have limited transmission when they are in the transparent state, typically about 25-30% of incident light. The resulting images are somewhat dim, but they can be produced to have a 150:1 ratio of desired transmission to unwanted transmission, which is acceptable.
When assessing the quality of 3-D motion picture images three figures of merit are used, namely maximum transmission, extinction ratio and effective contrast. Maximum transmission is the percentage of light generated by the projectors which actually reaches the eyes of a viewer. The extinction ratio is defined as a ratio of the brightness of a correct or wanted image to the brightness of an incorrect or unwanted image that leaks through the system. In a 3-D motion picture projection system, the extinction ratio gives an indication of how much ghosting a viewer will perceive. Effective contrast is a ratio which expresses the relative difference between the extreme whites and blacks which the projection system can reproduce at the eyes of a viewer. An image with a high effective contrast ratio is sharp with a stark difference between white and black. All three of the above figures of merit are important and must be considered to produce a high quality 3-D motion picture presentation.
It is among the objects of the invention to overcome the disadvantages of known methods of stereoscopic separation. A practical aim of the invention is to provide a method of stereoscopic image separation that allows acceptable levels of extinction ratio, maximum transmission and effective contrast without undue cost.